1. Field of the Invention
This invention relates to focus detecting devices of the TTL active type.
2. Description of the Prior Art
The TTL active type focus detecting device is shown in U.S. Pat. No. 4,357,085 (issued Nov. 2, 1982). Because such a device makes use of infrared light, or a light of a wavelength near this as in the luminous output of the metering source, when it is applied to cameras of which the objective lens is interchangeable, for example, single lens reflex cameras, a problem is produced which is shown below.
With the camera employing such a device, the in-focus position of the objective for photographic light (visible light) differs from that for projecting light (infrared light) due to chromatic aberration. In order to detect the in-focus position of the objective for photographic light, it is required that the position of the sensor for detecting the reflected light of the luminous output of the metering source be adjusted in accordance with the chromatic aberration of the objective, or that the output of the sensor be corrected electrically by a factor depending upon the chromatic aberration of the objective. For the objective having large chromatic aberration, such a correction must be made. Otherwise, accurate focus detection would be impossible. Meanwhile, different camera objectives have different chromatic aberrations. When applying such a device to cameras having interchangeable objectives, according to the prior art, each time the objective is attached to the camera, the value of the aforesaid correction factor had to be altered.
That is, common practice in the art is to provide individual objectives with respective different indicators such as pins representing the values of the correction factor for the photographic light relative to the projecting light. When the objective is attached to the camera, its pin either changes the sensor position, or gives necessary information to a signal processing circuit for the sensor output. The use of such a compensation method for chromatic aberration led to an increase in the complexity of the auto-focus camera structure employing such a device. Without the use of such indicators, the necessary compensating operation becomes extremely difficult to perform by hand.
Considering such situations, the present invention has been made, and its object is to provide a focus detecting device of the TTL active type which deletes the necessity of using the chromatic aberration indicator for the objective, while still making it possible to perform accurate focus detection even when the objective is interchanged.
Other objects of the invention will become apparent from the following description of embodiments thereof.